A Simple Fix For A Simple Circuit

I asked a student last week to throw together a quick breadboard circuit for testing power/volume relationships for speakers. I suggested the good ol’ LM386 from National Semi. This audio power amplifier has been around as long as I can remember, and the test circuit is about as simple as possible.  The next figure is straight from the data sheet:

The input is referenced to ground and the output is auto-cenetered around Vs/2.  Dual power supplies are not even needed. There are only six total parts, and the values are not all that critical. So I was completely surprised when the student came back later that day to say “it doesn’t work.”  I went to take a look and sure enough, this circuit just wouldn’t work. The output was distorted and clipped. There were strange noises coming out of the speaker that were dependent on the value of the output capacitor.

We spent a solid hour debugging the construction, trying new chips, trying new breadboards, etc. Nothing worked, and we left the lab rather defeated. The next morning, I was searching around some online forums and came across a discussion regarding the stability of this circuit. Then I remembered a sage engineer I used to work with who always insisted, “add decap when you’re using breadboards.” The inherent capacitance of the breadboard traces is rather large, and for feedback circuits, can cause all kinds of issues with supply coupling, etc. So I tried it, and sure enough, all was fixed!

We often incorrectly assume that when a circuit is connected to an expensive bench-top power supply, the supply is clean. Unfortunately, when a three foot cable, a bunch of wires, and a slew of coupling capacitors are added to the mix, this all falls apart. Simply adding one more capacitor between pins four and six is all it took. So in conclusion, this SEVEN part circuit works great and is still about as simple as they come. And speaking of decap, do a quick search for it online and notice the Sega game that comes up called “Decap Attack.” In Japan, it was supposedly released as “Magical Flying Hat Turbo Adventure.” Next time I run into decap issues, that’s how I’ll refer to it 🙂